Process Of GUI Design
Since the way the GUI behaves has a tremendous effect on how the entire application is designed, GUI design should be performed as early as possible - right after application requirements and general architecture were conceived.
GUI design includes functional requirements, and after that - screens design and flow design.
The participants in the process should be the system architect, the "marketing personnel" (i.e. those who know what users want/need), a GUI designer and a programmer. In smaller projects, all these functions might be performed by a single person - You.